Global warming potential (GWP; see Shine et al., 1990, for a formal definition)
is an index that attempts to integrate the overall climate impacts of a specific
action (e.g., emissions of CH4, NOx
or aerosols). It relates the impact of emissions of a gas to that of emission
of an equivalent mass of CO2. The duration of the perturbation
is included by integrating radiative forcing over a time horizon (e.g., standard
horizons for IPCC have been 20, 100, and 500 years). The time horizon thus includes
the cumulative climate change and the decay of the perturbation.

GWP has provided a convenient measure for policymakers to compare the relative
climate impacts of two different emissions. However, the basic definition of
GWP has flaws that make its use questionable, in particular, for aircraft emissions.
For example, impacts such as contrails may not be directly related to emissions
of a particular greenhouse gas. Also, indirect RF from O3
produced by NOx emissions is not linearly proportional to the amount of NOx
emitted but depends also on location and season. Essentially, the buildup and
radiative impact of short-lived gases and aerosols will depend on the location
and even the timing of their emissions. Furthermore, the GWP does not account
for an evolving atmosphere wherein the RF from a 1-ppm increase in CO2 is larger
today than in 2050 and the efficiency of NOx at producing
tropospheric O3 depends on concurrent pollution of the
troposphere.

In summary, GWPs were meant to compare emissions of long-lived, well-mixed
gases such as CO2, CH4, N2O,
and hydrofluorocarbons (HFC) for the current atmosphere; they are not adequate
to describe the climate impacts of aviation.

Nevertheless, some researchers have calculated a GWP, or modified version,
for aircraft NOx emissions via induced ozone perturbation (e.g., Michaelis,
1993; Fuglestvedt et al., 1996; Johnson and Derwent, 1996; Wuebbles, 1996).
The results vary widely as a result of model differences, varying scenarios
for NOx emission, and the ambiguous GWP definition for
short-lived gases. There is a basic impossibility of defining a GWP for "aircraft
NOx" because emissions during takeoff and landing would
have one GWP; those at cruise, another; those in polar winter, another; and
those in the upper tropical troposphere, yet another. Different chemical regimes
will produce different amounts of ozone for the same injection of NOx,
and the radiative forcing of that ozone perturbation will vary by location (Fuglesvedt
et al., 1999). In view of all these problems, we will not attempt to derive
GWP indices for aircraft emissions in this study. The history of radiative forcing,
calculated for the changing atmosphere, is a far better index of anthropogenic
climate change from different gases and aerosols than is GWP.

6.2.3. Alternative Indexing of Aviation's Climate Impact-RF Index

A new alternative index to measure the role of aviation in climate change is
introduced here: the radiative forcing index (RFI), which is defined as the
ratio of total radiative forcing to that from CO2 emissions
alone. Total radiative forcing induced by aircraft is the sum of all forcings,
including direct emissions (e.g., CO2, soot) and indirect
atmospheric responses (e.g., CH4, O3,
sulfate, contrails). RFI is a measure of the importance of aircraft-induced
climate change other than that from the release of fossil carbon alone. RFI
ranges between 2.2 and 3.4 for the various E- and F-type scenarios for subsonic
aviation and technical options considered here (see Section
6.6). Thus, aircraft-induced climate change with RFI > 1 highlights the
need for a thorough climate assessment of this sector as performed here. For
comparison, in the IS92a scenario the RFI for all human activities is about
1; for greenhouse gases alone, it is about 1.5, and it is even higher for sectors
that emit CH4 and N2O without
significant fossil fuel use.